Update thumbnail style only when they are visible (to improve scrolling through large documents)

This PR postpones highlighting the current page in ```PDFThumbnailViewer.scrollThumbnailIntoView()``` until the thumbnail view is opened. Setting the style of thumbnails can take long and become a bottleneck if the document (and thus, the DOM tree) is very large. Scrolling through very large documents is now smoother with this PR.

Good tests: 
[1] [pdf](https://www.bluetooth.org/DocMan/handlers/DownloadDoc.ashx?doc_id=282159) from #5207 and
[2] [pdf](http://www.etsi.org/deliver/etsi_ts/151000_151099/15101001/04.03.00_60/ts_15101001v040300p.pdf) from https://bugzilla.mozilla.org/show_bug.cgi?id=885287#c6. 

*A benchmark*: Scrolling through the first 50 pages of [2] with the ```↓``` key took 1:50 mins before the patch, and 1:07 after the patch

*Another benchmark*: While scrolling through the first 10 pages of [1] with the ```↓``` key, the frame rates are now much more often at the 60 fps mark:
![zwischenablage03](https://cloud.githubusercontent.com/assets/4624768/5558516/fba44b8a-8d2a-11e4-8601-e1fa7ab618c4.png)
![zwischenablage04](https://cloud.githubusercontent.com/assets/4624768/5558519/1000545c-8d2b-11e4-8021-22581803bea7.png)